Noise limiting detector



Nov. 10, 1942. THOMPSON 2,301,648

NOISE LIMITING DETECTOR Filed Nov. 30. 1940 3m entor Patented Nov. 10, 1942 NOISE LIMITING DETECTOR Leland E. Thompson, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 30, 1940, Serial No. 367,883

Claims.

This invention relates to a noise limiting detector or limiter for modulated carrier wave receiving systems and the like, and has for its primary object to provide an improved noise limiting detector which requires no manual adjustment to receive signals of varying strength.

It has been found that in known forms of automatically adjustable noise limiter circuits, wherein the signal carrier Wave provides the voltage to regulate the limiter level, noise voltages may raise the limiter level to such a high degree that the circuit become inefiective or inoperative.

In accordance with the present invention, a noise limiting detector circuit is provided wherein limiter level control potentials, as well as automatic volume control potentials, are obtained from a point in the circuit which is limited as to signal amplitude. With this arrangement, noise voltages at the limited point are comparatively low and the action of the noise limiting circuit is effective for varying signal levels and under conditions of operation involving relatively wide and extreme variations in noise level.

This invention relates particularly to a type of receiving system as shown, described and claimed in my U. S. Patent No. 2,179,277 for Modulated carrier wave receiving system, wherein a signal detector having a balanced bridge type output circuit is maintained in a condition to pass and rectify received signals under conditions of varying signal strength, while limiting the output for noise impulses above any instantaneous value of the signal strength.

It is a further object of this invention to provide a noise limiting detector circuit having a balanced bridge output circuit and means connected therewith for limiting the signal output from said circuit in response to potentials derived from a limited point on the bridge output circuit, whereby the limiting level may be dependent on signal strength variations and independent of noise and interference voltages transmitted through the system.

It is a still further object of this invention to provide an improved noise limiting detector circuit from which automatic volume control potentials may be derived to establish a volume level substantially independent of noise and interference voltage peaks.

The invention will be further understood from the following description when considered in connection with the accompanying drawing, and its scope is pointed out in the appended claims.

In the drawing:

Figure l is a schematic circuit diagram of a noise limiting detector circuit embodying the invention, and

Figure 2 is a similar schematic circuit diagram of a modification of the circuit of Fig. 1, also embodying the invention.

Referring to Fig. l, a signal detector 5 is coupled through a transformer or other suitable coupling means 6 to the signal amplifying means of a receiving system which, in the present example, is an intermediate frequency amplifier stage I having an input transformer 8. The remainder of the input system is not shown for the sake of simplifying the drawing.

The signal input circuit ID of the amplifier stage I is connected with the signal input grid 9 and with an automatic volume control lead I l in which is provided a suitable filter [2. The amplifier cathode I3 is connected to chassis or ground l4 through a self bias resistor [5.

The output circuit l8 of the amplifier is connected through the tuned primary circuit IQ of the coupling device 6 to the positive terminal 20 of an anode potential supply source represented by the series connected resistor elements 2| and 22 connected between positive and negative potential supply leads 23 and 24, respectively. An intermediate terminal 25 is connected to-chassis or ground as indicated at 26. The supply circuit I8 for the intermediate frequency amplifier stage 1 is also provided with a series controlling resistor 21 and a bypass capacitor 28.

The detector 5 and the balanced bridge output circuit therefor is substantially the same as shown in my aforesaid patent, comprising a pair of diode rectifiers, preferably in a common envelope as shown, and connected substantially in parallel between the tuned secondary circuit of the transformer 6 and the bridge output network indicated at 3|.

In the present example, the diode anodes 32 and 33 are connected in parallel to the high potential side of the circuit 3|], while the corresponding cathodes 34 and 35 are connected, respectively, through similar branch circuits 36 and 31, to a common terminal 38 and the low potential side of the tuned circuit 30. The branch circuit 35 includes two series resistors 39 and 40, while the branch circuit 31 includes similar series connected resistors M and 42, the latter preferably being adjustable, as indicated, by the contact 43. The latter contact is connected with the negative supply lead 24 and the series connection therefor with the resistor section 4| is completed through a lead 4 3 and a noise level control potentiometer device 45 which is connected across the potential supply leads 23, 2a in series with the resistor 27.

By adjusting the position of the potentiometer contact 56, a predetermined. positive potential may be applied to the cathode 35 with respect to the anode 33 of one of the diode rectifiers to provide a limiting level bias potential therefor, preventing the flow of current through the branch circuit 3? at or below the predetermined potential in response to an applied signal. Signals are derived from the circuits 3t and 3! at substantially equal potential points 58 and 26, respectively, as a balanced bridge output circuit for the rectifier device 5, to prevent the transmission of signals from the rectifier above a predetermined signal or noise level, this level in the present example being established by adjustment of the contact it as above described.

The output circuit for the terminal com,- prises an output lead 5i coupled to the control grid 52 of an audio frequency amplifier tube 53 through a coupling capacitor 55. The cathode 5 5 of the amplifier device 53 is connected to ground 56 through a self bias resistor 57 and the control grid receives bias therefrom through a suitable grid resistor 58, connected between the grid 52 and ground 58.

The circuit arrangement is such that in response to signal or noise voltages above a peak value determined by the adjustment of the contact d6 of the noise limiter control device, signal current from the diode current 35 flows through both of the branch circuits 36 and 37, establishing across the resistors lit) and 52 audio frequency signal potentials which are equalized by adjustment of the contact 53, thereby preventing the application of signals to the amplifier 53 and preventing the transmission of noise and interference voltage impulses to the output circuit 68 of the audio frequency amplifier. As an aid in balancing the bridge output circuit of the rectiher 5, suitable adjustable capacitors El, 612 and 63 are provided for-controlling the phase relation of the currents in the branch circuits.

Further, in accordance with the invention, automatic volume control potentials for the signal receiving system, and control potentials for the limiter circuit are derived from the balanced bridge output circuit between the limited point 59 and ground, by means of an automatic volume control amplifier tube @5 having a control grid 66 connected to the output lead 5! through a time delay filter comprising a resistor ti? and a suitable by-pass capacitor 68.

The cathode 59 of the amplifier 65 is connected to the resistor 22 as a source of biasing potential, through the contact 76 thereon, as shown, whereby in the absence of signals a predetermined negative biasing potential with respect to the cathode may be applied to the grid 65. The output anode circuit ii of the amplifier 65 includes a resistor i2 connected through ground 13 with the ground terminal which is more positive than the, terminal id, thereby providing anode current through the resistor it which operates as a source of biasing potential for the receiving system responsive to signal strength variations. For this purpose the AVG supply lead i i for the intermediat frequency amplifier is connected to the negative terminal is of the source 72 and likewise an AVC supply lead 15 for other portions of the receivingsystem, not shown, is connected therewith through a time delay filter comprising a series resistor l6 and by-pass capacitor 11.

In the absence of signals, bias potential on the AVG control amplifier 65 is adjusted by means of the contact if? to reduce or substantially cut off the plate current through the resistor 12. An 'ncoming signal then applies to the grid 66, across the resistor td, an increasing positive potential which causes the anode current to increase through the resistor '52, thereby increasing the negative bias applied to the AVG system such as "to the circuits H and 75 and to the control grid e, thereby reducing the gain in the I.-F. amplifier and other portion of the system.

As the gain of the amplifier 7 is reduced, the

anode current through the circuit i3 and series resistor Z? is likewise reduced. This has the effect of reducing the potential drop through the resistor 2? and therefore increasing the potential applied across the resistor d5 which is the limiter level control potentiometer. Thus, the voltage at the adjustable terminal 46 is increased in a positive direction, thereby increasing the positive potential on the cathode 35 with respect to the anode 3-3 of the diode rectifier 5, thereby raising the delay level in accordance with the signal strength variation or increase.

I In this system noise and interference peak voltages are limited before application to the AVG circuit while the signal cannot become limited because of the efficiency of the AVC system which has a relatively fiat characteristic. Furthermore, after the initial adjustment of the contact 46, the limiter level control device 'does not require further adjustment because of the controlling action of the amplifier current through the resistor 21, to change the limiter level with change in signal strength.

A modification of the noise limiting detector circuit to follow signal strength variations with the limiting action, while at all times providing effective limiting action in response to noise and interference peak voltage, is shown in Fig. 2, to which attention is now directed.

In the circuit of Fig. 2, the diode rectifier 5 is coupled, as in Fig. l, to the signal amplifying channel through the tuned input transformer 6, and comprises the parallel connected diode anodes 32 and 33 with the cathodes 3d and 35, respectively, connected in the branches 36 and 3! of a bridge output network 3 i. In the present example, however, the branch 56 is provided by a single potentiometer resistor 39 having a movable contact Si as the limited point and output connection for the networlnto which is connected an output lead 82 with an audio frequency amplifier 53 coupled thereto as in the preceding example for the derivation of audio frequency signals, the same reference numerals being applied, as in the preceding figure, to corresponding circuit elements.

The branch circuit 37 comprises a resistor section 83 connected between the terminal 33am} ground 3:; while the remainder of the branch circuit includes a second resistor 35 connected between ground 83 and both the cathode 35 and a cathode ill of a control amplifiertt. Variations in anode current through the device 88 and the cathode resistor from an adjustable source 69-99 is effective to vary the delay bias applied to the cathode 35 with respect tothe anode 33 of the rectifier 5. This variation is placed under control of the-signal at the limited point 8!, through a, connection 9! with a control grid 92 in the control device 88, a suitable time delay filter being provided in the circuit as indicated by the filter resistor 93 and the filter capacitor 94.

In operation, the limited point or contact 8| is adjusted to provide a balanced output circuit between ground 84 and the contact 8| so that with substantially no current fiow in the resistor 85 as in the absence of signals or on peak noise impulses, the bridge output circuit is balanced to prevent the application of signals to the amplifier 53 and the output circuit therefor. The contact 89 is then adjusted to apply sufficient anode current to the device 88 to establish a potential drop in the resistor 85 for the required delay potential on the diode cathode 35 above which peak noise and interference potentials are to be cut ofi in the presence of weak or substantially zero signal potentials.

As an applied signal is received in the diode rectifier circuit and increases in strength, the

potential applied to the grid 92 of the control amplifier 88 increases in a positive direction with respect to its cathode and causes an increase in the potential drop through the resistor 85 in the bridge network, thereby raising the limiter level automatically in response to an increase in signal strength. Thus, with this arrangement, normal slow variations in signal strength do not cause operation of the limiter, while rapid variations in the applied voltages as by interference of noise impulses, causes limiting as the delay bias is overcome.

From the foregoing, it will be seen that a normal increase in signal strength produces an increased limiter level so that the signal is not limited and a desired automatically adjustable limiter level is obtained. Since the control voltage is derived from a limited point 8| in the balanced bridge network or output circuit of the diode rectifier, noise peaks are limited and received noise or interference voltages do not operate to increase the limiter level as is the case in known limiter circuits.

I claim as my invention:

1. A signal amplitude limiter having a balanced output network, means for establishing a condition of unbalance in said network to permit signal flow therethrough, means providing a connection point on said network at which the signal amplitude is limited, and means for deriving from said connection point on said network a control potential for said first named means for varying the degree of unbalance of said network.

2. A signal amplitude limiter having a balanced output network, means for establishing a condition of unbalance in said network to permit signal flow therethrough, means for deriving from a limited point on said network a control potential, and means responsive to said potential for varying the degree of unbalance of said network.

3. A noise limiting detector comprising, in combination, a signal input circuit, a pair of rectifier elements connected therewith in parallel, an output circuit for said rectifier elements providing a balanced bridge network, means connected in one arm of said bridge network for applying a controlling potential to one of said rectifier elements, and means for deriving a controlling potential for said last named means from a limited point on said network.

4. A noise limiting detector comprising, in combination, a signal input circuit, a pair of rectifier elements connected therewith in parallel, an output circuit for said rectifier elements providing a balanced bridge network, means connected in one arm of said bridge network for applying a controlling potential to one of said rectifier elements, and means for deriving a controlling potential for said last named means from a limited point on said network comprising an amplifier having a control grid connected with said limited point and having an output impedance providing said first named potential.

5. A noise limiting detector comprising, in combination, a pair of rectifier elements having a balanced signal output circuit, means for effecting a predetermined unbalance in said network for the transmission of signals therethrough, and means responsive to signal potentials at a signal amplitude limited point on said network for determining the limiting leve1 of said detector.

LELAND E. THOMPSON. 

